1. Field of the Invention
The present invention relates to fuel cells and in particular to polymer electrolyte type hydrogen-oxygen fuel cells containing a polymer electrolyte.
2. Description of Related Art
Fuel cells containing a polymer electrolyte generally have the construction which comprises an integral an ion-exchange membrane as an ionic conductor and an oxygen electrode and a hydrogen electrode provided respectively on both the sides of the ion-exchange membrane as disclosed in "HYOMEN ( Surface )", Vol. 26, No. 12 (1988), pp. 904-909.
Furthermore, as mentioned in "DENKIKAGAKU (Electroche-mistry)", Vol. 53, No. 10 (1985), pp. 812-817, the two electrodes may comprise a platinum catalyst, carbon as a carrier for the catalyst and polytetrafluoroethylene (hereinafter referred to as "PTFE" as a binder and the same material as of the ion-exchange membrane.
The reactions in the fuel cells are mainly as follows:
(1) Diffusion of hydrogen and oxygen onto the surface of the catalyst.
(2) Reaction on the surface of the catalyst (in both the hydrogen electrode and the oxygen electrode).
(3) Conduction of protons (in the hydrogen electrode and in the ion-exchange membrane).
The diffusion or reaction rate in respective processes has a great influence on the cell output. To use a corrugated current collector for efficient diffusion of hydrogen and oxygen onto the surface of the catalyst in the above (1) is proposed in Japanese Patent Kokai (Laid-Open) No. 58-204188 (FIG. 2), No. 60-35472 (FIG. 1) and No. 55-113272. Furthermore, use of a carbon plate having rectangular grooves is shown in Japanese Patent Kokai (Laid-Open) Nos. 2-260371, 3-102774 and 2-86071.
When the corrugated current collector or carbon plate having rectangular grooves is brought into contact with the electrode, spaces are formed between the current collector or carbon plate and the electrode, and hydrogen or oxygen diffuses onto the surface of the electrode through these spaces. The above-mentioned construction is usually employed for fuel cells in which ion-exchange resins are used, and the output can be developed to some extent, but the above construction is merely for accelerating the diffusion of the gas onto the surface of the catalyst from the main stream of the fuel gas.
However, the diffusion of the gas also occurs in the electrode and the higher output will be able to be developed by carrying out the diffusion of the gas in the electrode efficiently. For efficient diffusion of the gas in the electrode, Japanese Patent Kokai (Laid-Open) No. 1-143151 proposes to increase the porosity of the electrode thereby to enhance the contact effect between the catalyst and the reaction gas and to increase the reaction rate.
The above conventional methods suffer from the following problems. The protons which have transferred through the ion-exchange membrane react with oxygen at a high rate at the interface between the ion-exchange membrane and the oxygen electrode. Therefore, water is produced at the oxygen electrode and a wafer film is formed at a high current density and results in a so-called flooding phenomenon. This water film causes reduction in the contact efficiency between the catalyst and the oxygen gas which has diffused through he electrode and the output density decreases. Thus, the cell performance becomes unstable. This phenomenon readily occurs at the interface between the oxygen electrode and the ion-exchange membrane, the efficiency of transferring protons from the ion-exchange membrane to the oxygen electrode also decreases to cause deterioration of the cell performance.